1. Field of the Invention
This invention relates to an instrument for monitoring light emission, particularly but not solely used in hygiene or biomass monitoring using the adenosine triphosphate dependent bioluminescent firefly luciferin/luciferase reaction.
2. Related Background Art
Luminometers for detecting the emission of light from a test sample are widely used to measure chemical and biological parameters in various types of samples by use of reagents which give rise to light through bioluminescent or chemiluminescent based reactions.
It is known to test for bacteria or other living cells by taking a sample and releasing adenosine triphosphate (ATP) contained within the cells using an appropriate reagent. Being a chemical compound common to all living cells, ATP is found in almost all foodstuffs of biological origin, within intact cells or as extracellular ATP. The level of ATP in a sample of a surface or rinse water may therefore be used as an indication of hygiene after cleaning, indicating the level of residues of food and/or microorganisms. The level of ATP can also be used in process and waste water, water tanks and cooling towers as a measure of biomass to follow the effect of biocide treatment. Many other applications of ATP bioluminescence are known conditions.
Bioluminescence monitors are well known. One such monitor is disclosed in WO90/04775 and comprises a chamber into which a elongate vessel containing the reagents can be inserted. A light detector, such as a photomultiplier tube or photodiode is disposed at the bottom of the chamber for measuring the amount of light which is emitted from the sample. The output of the light detector is connected to an electronic circuit which converts the measured light output to provide an indication of hygiene.
It will be appreciated that the monitor needs to be calibrated so that a correct indication of hygiene is provided according to the level of light which is output from the sample. Initially, this is performed in the factory where the monitor is produced using a sample which is inserted into an accurately calibrated test instrument and comparing the reading against the reading taken from the instrument under test using the same test sample. The monitor under test can then be adjusted to provide the same output as the calibrated test instrument.
It will be appreciated that it is desirable to check the calibration of the monitor on a regular basis. Hitherto this has been achieved by returning the monitor to a calibration center where the above-mentioned calibration procedure can be repeated.
In order to avoid the inconvenience of having to return the monitor to a calibration center, it has been proposed to provide users with a calibration checking device which emits a known amount of light. This device can be inserted into the monitor to check that the monitor measures the correct level of light.
A disadvantage of this is that the calibration checking device comprises plastics scintillant and a radio active light emitting material, such as carbon 14, which is potentially hazardous to the user. Also, the level of the light output of the calibration device falls as the scintillant material decays over time and thus an inaccurate calibration reading is provided. Another disadvantage is that the wavelength of the light output by the scintillant is substantially different from that of an actual sample.
It is also known to provide a monitor having a built-in calibration device comprising a light emitting diode (LED) or other light source directed at the photodetector. Whilst the wavelength of the light output by an LED is closer to that of an actual sample, the light output by an actual sample is very low, and the monitor has to be calibrated at this low level where even individual photons can be detected. Thus, the output of the LED has be to substantially attenuated, for example by placing a large block of semi-opaque plastics material between the LED and the photodetector.
Generally, calibration is performed by taking a reading over a period of 10 seconds or so. A disadvantage of this is that the output of the LED varies as it heats up over the calibration period, with the result that inaccuracies occur.
An other disadvantage of using an LED is that the light emitted is continuous, unlike the output of an actual test sample in which photons are randomly emitted.